Arabinogalactan (AG) and
lipoarabinomannan (LAM) are the two major
cell wall (
lipo)
polysaccharides of
mycobacteria. They share arabinan chains made of linear segments of alpha-1,5-linked D-Araf residues with some alpha-1,3-branching, the
biosynthesis of which offers opportunities for new
chemotherapeutics. In search of the missing arabinofuranosyltransferases (AraTs) responsible for the formation of the arabinan domains of AG and LAM in
Mycobacterium tuberculosis, we identified Rv0236c (AftD) as a putative membrane-associated polyprenyl-dependent
glycosyltransferase. AftD is 1400
amino acid-long, making it the largest predicted
glycosyltransferase of its class in the
M. tuberculosis genome.
Assays using cell-free extracts from recombinant
Mycobacterium smegmatis and
Corynebacterium glutamicum strains expressing different levels of aftD indicated that this gene encodes a functional AraT with alpha-1,3-branching activity on linear alpha-1,5-linked neoglycolipid acceptors in vitro. The disruption of aftD in
M. smegmatis resulted in
cell death and a decrease in its activity caused defects in
cell division, reduced growth, alteration of colonial morphology, and accumulation of
trehalose dimycolates in the
cell envelope.
Overexpression of aftD in
M. smegmatis, in contrast, induced the accumulation of two arabinosylated compounds with
carbohydrate backbones reminiscent of that of LAM and a degree of arabinosylation dependent on aftD expression levels. Altogether, our results thus indicate that AftD is an essential AraT involved in the synthesis of the arabinan domain of major
mycobacterial cell envelope (
lipo)
polysaccharides.
